A number of systems have been developed to avoid wiring stereo speakers directly to the source of signals used to drive the speakers, e.g., phonographs, tape decks, CD ROM players, or AM/FM tuners. By way of example, U.S. Pat. No. 4,829,570, issued to Larry Schotz on May 9, 1991, discloses a system of this type. This system, while not requiring direct wiring between the source of signals and the speaker, transmits the signals over the A.C. electrical conductors of the building in which the system is located. The signals transmitted in this manner are subject to certain undesirable effects, such as filtering for computer systems. This type of filtering may eliminate or degrade the signals intended for transmission to the speakers.
An alternative to using any form of wiring between a speaker and the source of signals for driving the speaker is to transmit the signals over the air via electromagnetic waves such as radio waves. This type of system requires the use of a transmitter for transmitting the signals, a receiver for receiving the signals at the speaker, and a power amplifier for amplifying the signals at the speakers to properly drive the speakers. The drawback with this type of system is that the FCC strictly regulates the frequencies at which information may be transmitted over the air without the requirement of an appropriate license. Additionally, the number of frequencies at which transmission may occur is limited. Currently, the frequency bands available for transmitting information using low power transmission without a license are at high frequency ranges. For example, the FCC currently allows the use of low power transmission (i.e., below 1 milliwatt, 47 CFR .sctn.15.249) in the range of 902 to 928 MHz, 2.4 to 2.483 GHz and 5.725 to 5.875 GHz.
Even though frequency bands are now available for the low power transmission of information at relatively high frequencies (over 900 MHz), current transmitter and receiver arrangements for transmitting at lower frequencies are not readily modifiable for the purpose of transmitting at relatively high frequencies such as over 900 MHz. Attempts to modify existing transmitter and receiver arrangements to transmit information have resulted in systems capable of transmitting information in a relatively crude manner. More specifically, the signal-to-noise ratio for signals transmitted using these modified systems is low, the frequency response of such systems is poor, the distortion is typically unacceptable, and it is difficult if not impossible to transmit multiple channels simultaneously. Additionally, microphonics is a problem when a receiver is built into a speaker to which it is providing signals. More specifically, conventional inductive loops, which determine the frequency of the receiver's local oscillator, are subject to the vibrations of the speaker. These vibrations may adversely modulate the frequency of the local oscillator.
Wireless speaker systems are desirable, since wiring is not required between the speakers and source of signals for driving the speakers; however, an arrangement of this type is not practical if the quality of the information signal driving the speaker is poor. Stereo speaker applications require high signal-to-noise ratios, good frequency response, low distortion, and stereo capability (simultaneous transmission of two channels of information) to be practical. A wireless speaker system will not be a replacement for a system using wires unless the quality of information signals provided to drive the speakers results in a sound at the speakers comparable with the sound at similar speakers in a system using wires.
Accordingly, the need exists for a system capable of transmitting and receiving signals over the air using a high frequency carrier and also maintaining the quality of the signals.